- Hit Dice measure attack ability and are rolled for hit points, which measure attack survivability; attack rolls, adjusted by hit dice in some way, determine whether you do damage or not.
- Damage is also rolled using dice; which dice and how much variation depends on the version.
- Level is a way of doling out Hit Dice and other abilities over time; there's some kind of advancement system, usually keyed to earning points from treasure and combat, with level increasing at certain break-points.
- Class is a way of trading away fast Hit Dice/combat ability advancement for another ability, like spells and thieving.
- There's also some general abilities not connected to class or level, mostly based on six ability scores (you know what they are.)
If you need advancement tables, steal them from one of those sources, or just assume Fighters need 2,000 to reach 2nd level, doubling the minimum every additional level; Clerics need 25% less, Magic-Users need 25% more, and Thieves need half what a Magic-User needs.
If you need a combat table, steal it from one of those sources, or use the one-page Target 20 system.
If you want to adjust some attacks or other actions based on the six ability scores, steal adjustment tables from one of those sources, or use my ability value table as a guideline to rolling your own; decide which situations deserve adjustments, whether the adjustments will work the same for all situations, and whether you will be giving a different modifier for each of the seven ranks (Extremely Low to Extremely High.)
That's all you need to know to run OD&D. Everything else is a specific implementation of this knowledge.